Refractories are classed chemically, as acid, basic or neutral; and by this arrangement, silica is the principal acid refractory. Bauxite, lime magnesite and dolomite are basic, while fireclay and chromite are neutral. Acid and basic materials in contact with each other readily slag and melt away. They cannot be used in contact with each other but may be used in the same furnace with a separating layer of neutral material.
Furnaces,— Metallurgical furnaces are di vided into two broad classes: first, those in which the heat comes directly from the combus tion of fuel; second, those in which electrical energy supplies the heat. Under class one fall the hearth furnaces, as the smith's forge; and the shaft furnace, as the blast furnace and the cupola. In these the fuel and charge to be heated are in direct contact. Then there are the reverbatory furnaces for roasting and smelting, where combustion takes place in a separate chamber and the charge is heated by the flame, also the closed vessel type of fur nace, as crucible, retort, muffle, etc., where the charge is inside sealed off from the source of heat which plays on the outside. Under this class also, come those furnaces which are merely containers, using no outside fuel, the heat coming from the charge itself, as the roasting kiln, bessemer convertor, etc. The electrical furnaces are classed as direct re sistance, the charge itself being a part of the electrical circuit or resistor; indirect resistance where the charge is surrounded by an electri cally heated resistor; direct arc furnaces, the charge forming one or both poles of an arc; indirect arc, the charge being in a space heated by an arc; arc resistance, where the charge is one pole of an arc and also a part of the elec trical circuit.
Ores.— The metals occur in nature either as pure native metal or in combination with other materials in an ore. Ore is defined as a metal bearing substance from which a metal, alloy or metallic compound can be extracted at a profit. It usually consists of. one or more economic minerals and a waste product or gangue. Ores are named according to their leading useful metal and are classified by the gangue as siliceous, calcareous, ferrugeneous; or broadly speaking, as acid and basic fluxing ores. Another classification refers to treat ment, as smelting or milling ores. The metals occur in nature as native metals, gold, silver, copper, mercury- as oxides, iron, tin; as sul phides, and under this class come selenides, tellurides, arsenides, antimonides; as halide.;, silver, mercury; salts of acids, carbonates. sili cates, etc. The largest number of commonly worked ores are in the oxide and sulphide classes.
Methods of Treatment.— It is here, by the treatment of the ores that we can divide metal lurgical methods into two broad classes: wet metallurgy and dry metallurgy. The wet method of treating an ore involves a leeching by a chemical solvent, followed by a means of separation of the required metal from solution. The dry method involves smelting and subse quent purifying methods by means of high temperatures. Considering dry metallurgy, the ore as it is dug out of the ground is seldom in a condition fit for smelting. Some ores may only require to be crushed to a uniform size, as the oxide ore of iron, haemitite; others must be heated to drive off moisture and volatile mat ter, called calcining; others must be roasted; still others are weathered. Calcination, or the driving off of volatile matter, at a low temperature is usually carried out in heaps, stalls or kilns. Calcining in heaps consists of building up a pile of alternate layers of wood or coal and ore to the height of five or six feet, then igniting at the bottom and letting it burn itself out. This process is usually carried on in yards where protection from the winds which will cause uneven burning, is afforded. The stall is the first improvement on the heap and consists of a four-walled enclosure, three permanent and the fourth built in after charg ing with fuel and ore. After the material is ignited the heat set free is sufficient to keep the process going. The process must he con tinuous and the losses of heat he kept at a minimum. This process is suitable only in ores which are rich in sulphur. A very large num ber of ores require a roasting preliminary to smelting. Roasting consists of heating the ore in contact with oxygen, chlorine, carbon, water vapor or sulphur, to a temperature below fusion or at most to incipient fusion with the purpose of driving off a volatile component. Roasts may be oxidizing, reducing, chloridizing ar sulphating, according to the ore and the product desired. The most common form of roast is the oxidizing roast of sulphide ores to eliminate the sulphur and if carried to completion leaves a metallic oxide of the metal for smelting. This is called dead or sweet roasting and re quires prolonged heating and much stirring. Otherwise, the sulphite is converted partially into oxide and partially into sulphates with some remaining sulphide. The chloridizing roast is used as a preliminary to a wet metallurgical process whereby a chloride is produced which is amenable to solvents. Roasting may he car ried out in heaps, stalls, shaft furnaces or re verbatory furnaces.